Biodegradation is the degradation and assimilation of organic polymers and other compounds by the action of living organisms. With a few notable exceptions, synthetic plastics are resistant to microbiological degradation, but naturally occurring polymers are susceptible to breakdown by fungi and bacteria. Biological degradation of polymers is facilitated by linearity of the polymer chains and by low molecular weights (below 1,000). These conclusions are supported by laboratory tests in which polymer samples are exposed to mixtures of known micro-organisms and by soil burial tests. Applications for biodegradable plastics in agriculture include slow release urea-formaldehyde fertilizers and degradable containers and matrices for the sustained release of pesticides and herbicides. Absorbable sutures are prominent examples of surgical applications for biodegradable plastics. Absorbable plastics have also been developed for use in orthopedic surgery in the form of screws, pins and plates.
Polycaprolactone polymer systems belong to the family of polymer of cyclic esters. In early 1930, the pioneering work in the area of polymerization was started by W. H. Carruthers.
In late 1950 and the early 1960's, Union Carbide Corporation recognized the usefulness of the above polymer systems and their applications in the molding, coating, fiber, film, and others. Their work resulted in various types of polymer systems and the economically feasible manufacturing systems for the manufacture of these polymer systems. E. F. Cox and F. Hostettler, of Union Carbide, received a number of patents in this field (U.S. Pat. Nos. 3,021,309-3,021,317).
In the late 1960's, in order to overcome a problem faced in the regeneration of forests by bare root planting methods, the U.S. government contacted Union Carbide Corporation to develop biodegradable plastic containers for use in lieu of previously used containers, which were manufactured of non-degradable plastic and surrounded the root systems of the seedlings, to the detriment of their growth. Union Carbide obtained a number of patents, as indicated below, which were directed to this problem. They were:
______________________________________ 3,844,987 3,923,729 3,850,862 3,921,333 3,850,863 3,929,937 3,852,913 3,931,068 3,867,324 3,932,319 3,919,163 ______________________________________
Each of these patents were related to the biodegradable feature of polycaprolactone. None of these patents contain any suggestion which would lead to the solution of the problems which we confronted in our attempts to develop a shotshell component.
Because of the great resistance of most plastics to environmental degradation, new needs for readily degradable plastics continue to arise. Also, known needs continue to increase or expand, merely because of the continually increased use of plastics by the public. One of the most recently publicized needs for readily degradable plastic is currently being stressed by environmental groups, because of the littering problem created by shotgun shells discharged by hunters. Thus, a need for plastic shotshell components, such as wads, shot-cups, and casings, has received much attention of late. When the hunting is conducted on land, such components rest upon the upper surface of the ground and present an unsightly view. When the shotguns are discharged over water, they tend to float and present other problems in addition to unsightliness. Our invention is directed toward overcoming this problem.
In addition to the need for overcoming the littering problem, there is a need to overcome a related problem, namely the problem created by the toxicity of the material from which shotshell components may be made. As a consequence, our attention was also directed toward the need to provide shotshell components which are nontoxic and will degrade within a reasonable length of time.
Although the above problems are of serious consequence, we also had an additional problem in mind, while seeking to overcome littering and toxicity. This additional problem was the need to provide an improved performance in the shot patterns of shotshells. This related need has arisen as a result of numerous states enacting laws which require the use of steel shot, instead of lead shot, in waterfowl hunting. The underlying reason for this requirement is that it has been well established that spent lead shot, which collect at the bottom of water bodies such as sloughs, become ingested by ducks and geese, which causes their deaths via lead poisoning. Although steel shot have been fairly well accepted, numerous complaints have arisen about their effectiveness, because of their lighter weight, as compared to lead shot. It is well established that steel shot carry a lesser impact upon striking the target because of such lighter weight and, as a consequence, there are many more wounded and unrecovered birds which would have been killed, if struck by the same number of lead shot. It is clear, therefore, that, if satisfactory and equal results are to be obtained in the use of steel shot, an improved pattern performance is necessary. Thus, while we were seeking to overcome toxicity and littering, we also sought to improve the pattern performance of steel shot by providing an improved shotshell component, such as a shot-cup which moves outwardly from the shotshell with the shot upon firing of the shotshell.
As indicated above, it is recognized that we need nontoxicity and rapid degradability, if we are to solve the littering and poisoning problems. The experience of the trade to date has been that polyethylene, particularly linear polyethylene, has been the most suitable plastic for use in the manufacture of shotshell components, such as casings, wads, and shot-cups. The reason for this is that polyethylene has the most suitable engineering properties and provides the most satisfying ballistic data. It has been previously known that polycaprolactone ##STR1## is nontoxic and is the most rapidly degrading plastic available, particularly in cold and wet climates and in water bodies. It appears that polycaprolactone components potentially will degrade rapidly, even in water bodies, such as fresh lakes, seas and other bodies of waters. Its use, however, has been prohibited, because its engineering properties are such that it alone cannot be used, since it has serious rigidity limitations at higher temperatures. Also, commercially available polycaprolactone systems did not yield trouble-free injection-molding processes for the molding of shotgun shell components. We reasoned that, if these two adverse aspects of polycaprolactone could be overcome, perhaps we could arrive at a suitable biodegradable shotshell component which would solve at least one of the above problems, namely littering.
Blending of polyethylene with polycaprolactone, however, is not feasible because the wide differences in their melting temperatures precludes injection-molding of an alloy or blend thereof. If the shot-cup is made of a polymer system consisting of linear polyethylene and the terpolymer, ethylene vinyl acetate carbon monoxide, it is photodegradable, but its degradability is very slow in wet and dark climates, and is not a suitable choice for the aforementioned environment as the degradable plastic.
The reason that polycaprolactone alone has heretofore been considered to be incapable of being injection-molded was that serious difficulty was experienced in obtaining the release of the shotshell components from the mold. We concluded that, if we could somehow modify polycaprolactone so as to obtain a satisfactory release of the parts from the mold, then we would have made progress toward overcoming at least one of its two adverse ammunition characteristics. In an effort to ameliorate this problem, we decided to follow two separate routes.
The first route we followed was to compound and repelletize the polycaprolactone polymer system, after incorporating a release additive package. We found, however, that although this appeared to ameliorate the problem, it lowers the molecular weight of the polycaprolactone polymer system to a substantial extent, and resulted in even further lowering the high temperature rigidity of this material. In view thereof, we felt compelled to discontinue following this approach.
The second route which we followed was to surface-coat the polycaprolactone polymer with liquid and solid lubricants to function as the releasing additive package. By doing so, we avoided the relatively extreme heating and cooling which was associated with the compounding and repelletizing procedure referred to above and, as a consequence, we saved the polycaprolactone from degrading further to yield maximum high temperature rigidity rating for the shotgun shell applications. We have apparently solved the problem in a much more satisfactory manner. We tried calcium stearate, zinc stearate, aluminum zinc stearate, aluminum stearate, and various fatty acid esters and fatty alcohol esters as elements of release additive packages. Although they functioned to a limited extent, none of these release additive packages functioned to a satisfactory degree. We found, however, that a combination of vegetable oil and fatty acid ester amides function in a highly satisfactory manner, if used advisedly, so that the polycaprolactone polymer system becomes readily capable of being injection-molded into the desired shotshell components.
A suitable shotshell component must perform adequately within the range of 150.degree. F. to -20.degree. F. Pure polycaprolactone shotshell components do not yield satisfactory ballistic performance at 150.degree. F. In an effort to ameliorate the high temperature rigidity problem, we chose two routes. The first route was to produce an alloy of polycaprolactone with rigid biochemically degradable plastics. In doing so, we worked with hydroxypropylcellulose, cellulose acetate, lactic acid polymers, and starch-based polymer systems. The hydroxypropylcellulose and starch-polymer system deteriorated the desired properties, and the polymer system became weaker than the initial polycaprolactone. The cellulose acetate and polylactic acid polymers did yield some improvement in that the high temperature rigidity of the system improved for the shotshell application, but simultaneously the low temperature limitations of the initial polycaprolactone increased. Work is being continued in this area.
The second route which we followed in seeking to ameliorate the high temperature rigidity problem was to rigidize and reinforce the polycaprolactone with inorganic fillers and organic fibers, such as cotton flock, talc, calcium carbonate, silica, clay and combinations thereof, and others. This approach failed to meet aforementioned shotgun shell performance, and the work in this area has been terminated.
A third approach is being practiced in a further effort to ameliorate the high temperature rigidity problem of the polycaprolactone polymer system. This approach is based upon a combination of the two above approaches, in which we seek to develop a suitable alloy.
Some notable alloying materials which we have explored, and the results of which are listed, is set forth hereinbelow:
(1) The alloying of various grades of starch-based polymer system, with no positive results. PA0 (2) The alloying of polycaprolactone with terpolymers, with no positive results. PA0 (3) The alloying of polycaprolactone with polylactide polymer systems, with positive results in terms of improvement of high temperature rigidity, but not satisfactory enough to meet total performance requirements. PA0 (1) Union Carbide Chemicals and Plastics Co., Inc., 39 Old Ridgebury Road, Danbury, Conn. 06817-0001, Telephone 1-800-223-0537. PA0 (2) Solvay Polymers, Inc., Technical Center, 1230 Battleground Road, P.O. Box 1000, Deer Park, Tex. 77536. PA0 (3) Daicel Chemical, 1 Tepo-Cho Sakai, Osaka-Pref, Osaka, 590, JAPAN, Telephone 81-6-722-273111.
In all, we have tested 10-15 different materials in combination with polycaprolactone and the alloys thereof, in our efforts to develop a suitable polycaprolactone polymer. To date, the best we have found is the polymer system described and claimed herein.